1. Field of the Invention
This invention relates to a solid-state deformation process, and more particularly to a process for solid-state extrusion of thermoplastic polymer billets into oriented large extruded shapes at high extrusion rates.
Most polymers have a chain of carbon to carbon bonds along their backbone. Upon solidification of some polymers, a portion of the polymer chains in the material are folded to form crystals which are randomly oriented within the material. If even a small portion of the material behaves in this manner, the material is referred to as semi-crystalline. Such material may deform easily by bending, sliding and breaking of the crystals or a small fraction of the non-crystalline entangled molecular chains If the chains are aligned or oriented, the mechanical strength is much improved. It is known that properties such as strength and stiffness are enhanced by aligning or orienting the polymer chains. One technique for orienting some polymers, such as polyethylene, is by plastic flow at temperatures below the melting point.
Processes for the solid state deformation of polymers are well known. Among the processing techniques used to make profiles of polymers are ram and hydrostatic extrusion. In ram extrusion the billet of polymer is placed inside a usually cylindrical pressure chamber, so that the surface of the billet is in immediate contact with the walls of the chamber. One end of the chamber is fitted with a die, whose opening corresponds to the profile it is desired to produce. The other end of the pressure chamber is closed by an axially mobile ram, attached to a hydraulic system, so arranged that the ram pushes against the billet of the polymer and forces the polymer out from the chamber by flowing through the die.
In hydrostatic extrusion on the other hand, the billet is much smaller than the pressure chamber, and the surface is separated by some distance from the chamber wall. The intervening space is filled with a hydraulic fluid. One end of the chamber is fitted with a pressure generating device, which may be a piston, or by an inlet through which hydraulic fluid is pumped into the chamber. The other end of the chamber is fitted with the die. One end of the billet is machined in such a way that the nose piece fits into the throat of the die, and makes a liquid tight seal. During extrusion, the pressure on the hydraulic fluid is increased. This pressure is transmitted in both the axial direction and the radial direction to the billet, so that it is pressurized equally in all directions. As a consequence, the surface of the billet is in contact with the oil, and some of this oil adheres to the surface of the billet as it passes through the die, providing a significant amount of lubrication.
During solid state deformation processes such as rolling, drawing and extrusion, the polymers typically lose the spherulitic morphology generated during cooling from the molten state, and become fibrillar with fibrils and these become oriented in a longitudinal direction. The orientation of the polymer in the longitudinal direction increases the mechanical properties of the polymer, e.g. its tensile strength and its tensile modulus. These are sought after properties.
2. Description of the Related Art
Various prior workers have extruded semi-crystalline polymers by the ram extrusion method. For instance, Kolbeck and Uhlman, Journal of Polymer Science: Polymer Physics Edition, Vol 15, 27-42 (1977), disclose the ram extrusion of semicrystalline polymers to produce up to 0.60 cm diameter rod and 1.14 cm wide by 0.04-0.13 cm thick sheet, at draw ratios from 9 to 36. However, the rate of ram advance in all instances was 0.34 cm/min which translates into a maximum extrusion speed of only 12 cm/min.
Imada et al, Journal of Materials Science Volume 6, 537-546 (1971) disclose the ram extrusion of high density polyethylene into thin rods from 2 to 4.5 mm diameter, from a billet of 9.9 mm in diameter to give a maximum draw ratio of 12.5, at an unreported speed.
Farrell and Keller, Journal of Materials Science Vol. 12, 966-974 (1977), disclose a ram extrusion process in which high density polyethylene was extruded into a 5 mm diameter fibre, at a haul off tension of 1 kg. The ram speed varied from 0.005 to 2.0 cm/min, which at the maximum draw ratio of 20 which was used translates into extrusion speeds up to 40 cm/min.
Bigg, Polymer Engineering and Science, Volume 28, 830-841 (1988) states that ram extrusion is very slow, and is limited to low molecular weight polymers. He also discloses that extrusion rates of only 2.5 mm/min have been reported for high density polyethylene extruded at an extrusion ratio of 40:1. Attempts to extrude at higher rates resulted in process instabilities.
Otto and Hower in GB Patent 2052357, disclose a method for ram extrusion, but the material extruded is a powder, and the extrudate is forced through the bore in which the ram travels and not through a die at a high draw ratio to produce oriented polymer profiles.
Dunnington et al in U.S. Pat. No. 4,266,919 disclose a rapid extrusion of polymer profiles ranging from 9.5 to 15.9 mm diameter at 40 cm/min, using a ram whose surface is made of a fluoropolymer material.
Groff in U.S. Pat. No. 4,145,175 discloses an extrusion apparatus for polyethylene operating at 350.degree.-500.degree. F. It uses a short stroke reciprocating punch as the ram to extrude polyethylene powder at a draw ratio of 1:1.
Murray in U.S. Pat. No. 3,954,372 discloses a device for the ram extrusion of polymer powders using a short stroke reciprocating ram. This patent is silent on temperature and draw ratios. This disclosure is for the extrusion of powders, not billets, by a reciprocating punch, and not a long stroke ram.
Cottington in U.S. Pat. No. 3,887,319 also discloses an apparatus for the extrusion of polymers, where a reciprocating short stroke ram is used to extrude granules of polymer, at room temperature at a maximum rate of 12.5 cm/min. with a draw ratio of 3:1.
All of these prior systems suffer from problems of very low rates of production and the ability to produce products of only small cross-sectional area, and provide little or no enhancement in properties through molecular orientation.